Fix inconsistencies between --all and --filter=all in volume prune

[simonbrauner]

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Does this PR introduce a user-facing change?

The `volume prune` options `--all` and `--filter=all` no longer has different behavior.

[packit-as-a-service]

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[simonbrauner]

While working on #28413, I found out that podman volume prune --filter=all=true outputs Error: "all" is an invalid volume filter, while podman volume prune --all=true does not, but according to the documentation, these options should be equivalent

[Honny1]

LGTM, Thanks.